1. Field of the Invention
The present invention relates to a method of axially aligning a charged particle beam and also to a charged particle beam system.
2. Description of Related Art
In recent years, scanning electron microscopes for making observations and measurements on fine structures of living organisms, materials, semiconductors, and so on and charged particle beam systems, such as metrology SEMs used for measurements on semiconductor device circuit patterns, have become known.
In a charged particle beam system, the profile of the charged particle beam (such as an electron beam) needs to have a good circular shape. In a charged particle beam system, the shape of the charged particle beam is corrected by performing astigmatic correction of the charged particle beam.
For example, JP-A-2007-180158 discloses a technique of performing astigmatic correction placing coils of a pair of coils on the opposite sides of an electron beam. If the deflecting forces applied from the opposing coils on the electron beam are out of balance, the beam will be deflected off the center of the optical axis when an astigmatic correction is performed. The deflection moves the field of view being observed. This will impair the observational operability when an astigmatic correction is made. Therefore, it is necessary to adjust the coil currents such that the deflecting forces applied on the electron beam are well balanced even if the values of the currents supplied to the opposing coils are made variable.
For example, JP-A-2007-180158 states that a centering operation (axial alignment of an electron beam) for bringing the position at which the magnetic field strength becomes null into coincidence with the position of the electron beam prior to an astigmatic correction such that the deflecting forces applied on the beam are balanced even if the values of the currents supplied to the opposing coils are made variable. In particular, JP-A-2007-180158 discloses a method of making axial alignment of an electron beam. In this method, the ratio of the currents fed to the opposing coils is varied in turn. At each ratio, the electron beam is scanned. The positions of marks are detected. Also, information about variations of the positions of the marks when the currents fed to the opposing coils are varied is detected. The ratio of the currents which minimizes the variations of the mark positions is regarded as an optimum value for the centering operation.
However, in the electron beam axial alignment method of JP-A-2007-180158, scanning of the electron beam and detection must be conducted repetitively until the centering operation gives rise to an optimum value. Hence, the centering operation is time-consuming to perform. In this way, it may not be easy to perform axial alignment of the electron beam.